The organic EL light-emitting device is a highly promising self luminous element for use as a video display device such as a display and a surface light source. The organic EL light-emitting device is used for a video display device either as a part color system to emit a monochromatic light or as a full color system having areas to emit light in the three primary colors of red (R), green (G) and blue (B). The organic EL light-emitting device used as a surface light source is configured as a thin-film device.
This organic EL light-emitting device is fabricated generally by laminating a transparent electrode constituting an anode, an organic EL layer and a metal electrode constituting a cathode in that order on a transparent substrate such as a glass substrate. By the voltage applied between the transparent electrode and the metal electrode, the electrons supplied from the cathode and the holes supplied from the anode are recombined in the organic EL layer. With the recombined, excitons are generated and during the transition of the excitons from the excited state to the normal state, the EL light is emitted. The EL light thus emitted is transmitted through the transparent electrode and extracted externally from the transparent substrate.
The light extraction efficiency of this organic EL light-emitting device is expected to be low. Specifically, the refractive index of ITO (indium tin oxide) used as the transparent electrode is about 2.0 higher than the refractive index 1.5 of the glass substrate used as the transparent substrate. Therefore, the greater proportion of the light proceeding toward the glass substrate from the transparent electrode becomes a transparent electrode waveguide mode propagates through the neighborhood of the transparent electrode and fails to be radiated to the glass substrate from the transparent electrode. FIG. 1 shows the result of simulation of the electric field distribution in the transparent electrode waveguide mode trapped in the transparent electrode. In FIG. 1, in accordance with the distance from the metal electrode, the refractive index distributions of the ITO and the glass substrate are indicated by dashed lines following Alq3 and PVK making up an organic EL layer, while the field intensity in the transparent electrode waveguide mode of the light having the light emission wavelength of 524 nm is indicated by solid line. As understood from FIG. 1, although the exudation of about the effective wavelength is observed, the transparent electrode waveguide mode is trapped by the ITO high in refractive index and cannot be extracted externally.
Further, as compared with the refractive index 1.0 of air, the refractive index of the glass substrate is as high as about 1.5. Therefore, the transparent substrate waveguide mode prevails, in which the greater proportion of the light proceeding toward the glass substrate from the transparent electrode propagates through the glass substrate and fails to be radiated into the air from the glass substrate. As a result, the greater proportion of the light emitted from the organic EL layer assumes the transparent electrode waveguide mode or the transparent substrate waveguide mode, thereby reducing the light extraction efficiency.
Incidentally, in this application, the light extraction efficiency is defined as the ratio of photons capable of being extracted out of the organic EL light-emitting device to the photons emitted as light from the organic EL layer.
Also, in this application, the waveguide mode is defined as the state of the electromagnetic wave propagating through a waveguide, and the radiation mode is defined as the state of the electromagnetic wave not locally existent in the waveguide.
The actual light extraction efficiency is difficult to measure. For calculation thereof, therefore, simulation is unavoidably resorted to. In view of the fact that the thickness of the transparent electrode and the organic EL layer are equal to or smaller than the effective wavelength of the light emitted from the organic EL layer, and a simple method using the geometrical optics is known to develop a considerable error. In addition to the geometrical optics, therefore, various calculation methods have been tried. As the result of simulation using the finite time domain difference method, the present inventors have made it clear that even in the case where the thickness of the transparent electrode is changed from 50 nm to 200 nm and the thickness of the organic EL layer from 20 nm to 80 nm, the transparent electrode waveguide mode remains about 40 to 50% and the transparent substrate waveguide mode about 25 to 35% of the light emitted from the organic EL layer, so that the light extraction efficiency of the light emitted from the glass substrate is about 15 to 30%.
Incidentally, in this application, the effective wavelength is defined as the wavelength of the light in a propagation medium, and expressed asEffective wavelength=wavelength in vacuum/refractive index of propagation medium
In the prior art, as a method to improve the light extraction efficiency of the organic EL light-emitting device, a technique is disclosed in which a condensing lens is arranged in the boundary between the transparent electrode and the transparent substrate (See Patent Document 1, for example). The prior art with the condensing lens arranged in the boundary between the transparent electrode and the transparent substrate is shown in FIG. 2. Numeral 81 denotes a glass substrate, 82 a transparent electrode, 83 an organic EL layer and 84 a condensing lens. This has such a structure that the angle of incidence of that part of the light emitted from the organic EL layer 83 which is totally reflected is converted into a small angle by a plurality of condensing lenses 84 providing a light angle conversion means thereby to extract the light.
As shown in FIG. 2, however, in the case where the condensing lens 84 formed on the upper surface of the glass substrate 81 is used, the ratio of the light from the organic EL layer 83 immediately under the center of the condensing lens 84 (point A in FIG. 2) which is totally reflected can be decreased, whereas the ratio of the light from the organic EL layer 83 not immediately under the lens center (point B in FIG. 2) which is totally reflected is rather increased. (Patent Document 1: Japanese laid-open Publication No. 2002-260845)